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Hello,

I have been driving my tachometer off the distributor in my 1974 911 with a 3-pin CDI+. My tach started to act up with some minor bouncing the other day. I decided to check the points trigger signal to the CDI+ with my Picoscope and noticed it has an 8v square wave signal to the box and tach. Is this the normal signal for a 3-pin CDI+?  I always thought the later tachs need an 11+ volt signal. 
 

Any input would be helpful!

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It's a common misconception that the tachos are 'driven' from a voltage signal.  The opposite is true, the tacho input pin is pulled up to a voltage and the points or the CDI box GROUNDs it to create the square wave input.   For this reason, the voltage observed depends on the tacho end, not the CDI or points end.    It kind of depends on the tacho and the electronics inside it.  8v is fairly common.

I would check your alternator/battery voltage as tacho weirdness can indicate a faulty voltage regulator.

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